This study is concerned with: (1) the mechanism involved in the transmural passage of blood cells from their extravascular site of formation into the peripheral pool; (2) the disturbance in this passage in myelogenous leukemia and the possible underlying mechanism of this disturbance; (3) the entry of malignant leukemia cells originating from a subcutaneous tumor into the circulation; (4)\the entry into the circulation of cells of malignant tumors, i.e., with the first phase of metastasis formation; (5)\the subsequent organ disposition of the intravascular malignant cells; (6)\the surface characteristics and changes in the surface characteristics of the cellular elements involved in these processes. The methodology includes: transmission electron microscopy; scanning electron microscopy by imaging through secondary electrons and backscattered electrons; the tracing of malignant leukemia cells by labeling by diaminoacridines; the use of cell surface-reactive agents on the exposed and isolated fixed and living endothelial cell surface of the vessels involved.